Messages in this thread Patches in this message | | | Date | Fri, 12 Sep 2008 22:16:59 +0300 | From | Török Edwin <> | Subject | mmap/munmap latency on multithreaded apps, because pagefaults hold mmap_sem during disk read |
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On 2008-09-08 22:10, Török Edwin wrote: > [snip] > There is however a problem with mmap [mmap with N threads is as slow as > mmap with 1 thread, i.e. it is sequential :(], pagefaults and disk I/O, > I think I am hitting the problem described in this thread (2 years ago!) > http://lwn.net/Articles/200215/ > http://lkml.org/lkml/2006/9/19/260 > > It looks like such a patch is still not part of 2.6.27, what happened to it? > I will see if that patch applies to 2.6.27, and will rerun my test with > that patch applied too. >
The patch doesn't apply to 2.6.27-rc6, I tried manually applying the patch. There have been many changes since 2.6.18 (like replacing find_get_page with find_lock_page, filemap returning VM_FAULT codes, etc.). I have probably done something wrong, because the resulting kernel won't boot: I get abnormal exits and random sigbus during boot.
Can you please help porting the patch to 2.6.27-rc6? I have attached my 2 attempts at the end of this mail.
Also it looks like the original patch just releases the mmap_sem if there is lock contention on the page, but keeps mmap_sem during read? I would like mmap_sem be released during disk I/O too.
I also tried changing i_mmap_lock into a semaphore, however I that won't work since some users of i_mmap_lock can't sleep. Taking the i_mmap_lock spinlock in filemap fault is also not possible, since we would sleep while holding a spinlock.
Just to confirm that the problem is with pagefaults and mmap, I dropped the mmap_sem in filemap_fault, and then I got same performance in my testprogram for mmap and read. Of course this is totally unsafe, because the mapping could change at any time.
> [2] the test program is available here: > http://edwintorok.googlepages.com/scalability.tar.gz > You just build it using 'make' (has to be GNU make), and the run > $ sh ./runtest.sh /usr/bin/ | tee log > $ sh ./postproc.sh log
I've written a latency tracer (using ptrace), and I identified the mutex/mmap related latencies (total runtime 23m): - mmap-ed files (created by libclamav) ~6680 ms total - creating/removing anonymous mappings, created by glibc, when I use functions like fopen/fclose:
With 8 threads: =====> Total: 3227.732 ms, average: 3.590 ms, times: 899 === /lib/libc.so.6 (mmap) === /lib/libc.so.6 (_IO_file_doallocate) === /lib/libc.so.6 (_IO_doallocbuf) === /lib/libc.so.6 (_IO_file_seekoff) === /lib/libc.so.6 (_IO_file_attach) === /lib/libc.so.6 (_IO_fdopen) === /usr/lib/libz.so.1 (gzflush) === /usr/lib/libz.so.1 (gzdopen) === /usr/local/lib/libclamav.so.5 libclamav/scanners.c:470 (cli_scangzip)
=====> Total: 2069.519 ms, average: 3.624 ms, times: 571 === /lib/libc.so.6 (munmap) === /lib/libc.so.6 (_IO_setb) === /lib/libc.so.6 (_IO_file_close_it) === /lib/libc.so.6 (_IO_fclose) === /usr/lib/libz.so.1 (gzerror) === /usr/local/lib/libclamav.so.5 libclamav/scanners.c:529 (cli_scangzip)
with 4 threads: =====> Total: 578.607 ms, average: 4.743 ms, times: 122 === /lib/libc.so.6 (munmap) === /lib/libc.so.6 (_IO_setb) === /lib/libc.so.6 (_IO_file_close_it) === /lib/libc.so.6 (_IO_fclose) === /usr/lib/libz.so.1 (gzerror) === /usr/local/lib/libclamav.so.5 libclamav/scanners.c:529 (cli_scangzip)
=====> Total: 148.083 ms, average: 2.278 ms, times: 65 === /lib/libc.so.6 (mmap) === /lib/libc.so.6 (_IO_file_doallocate) === /lib/libc.so.6 (_IO_doallocbuf) === /lib/libc.so.6 (_IO_file_seekoff) === /lib/libc.so.6 (_IO_file_attach) === /lib/libc.so.6 (_IO_fdopen) === /usr/lib/libz.so.1 (gzflush) === /usr/lib/libz.so.1 (gzdopen) === /usr/local/lib/libclamav.so.5 libclamav/scanners.c:470 (cli_scangzip)
With 8 threads situation is much worse than with 4 threads even for functions using anonymous mappings.
Of course the latency tracer has its own overhead (1.2 ms average, 67 ms max for 8 threads, and 0.1 ms average, 31 ms max for 4 threads),but these latencies are above that value.
Best regards, --Edwin
--- First attempt: arch/x86/mm/fault.c | 33 +++++++++++++++++++++++++++------ include/linux/mm.h | 1 + include/linux/mm_types.h | 1 - include/linux/sched.h | 1 + mm/filemap.c | 18 ++++++++++++------ 5 files changed, 41 insertions(+), 13 deletions(-) diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index 455f3fe..38bea4b 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -578,10 +578,7 @@ int show_unhandled_signals = 1; * and the problem, and then passes it off to one of the appropriate * routines. */ -#ifdef CONFIG_X86_64 -asmlinkage -#endif -void __kprobes do_page_fault(struct pt_regs *regs, unsigned long error_code) +static inline void __do_page_fault(struct pt_regs *regs, unsigned long error_code) { struct task_struct *tsk; struct mm_struct *mm; @@ -702,6 +699,7 @@ again: down_read(&mm->mmap_sem); } +retry: vma = find_vma(mm, address); if (!vma) goto bad_area; @@ -761,8 +759,21 @@ survive: } if (fault & VM_FAULT_MAJOR) tsk->maj_flt++; - else - tsk->min_flt++; + else { + if ((fault & VM_FAULT_RETRY) && (current->flags & PF_FAULT_MAYRETRY)) { + current->flags &= ~PF_FAULT_MAYRETRY; + goto retry; + } + /* + * If we had to retry (PF_FAULT_MAYRETRY cleared), then + * the page originally wasn't up to date before the + * retry, but now it is. + */ + if (!(current->flags & PF_FAULT_MAYRETRY)) + tsk->maj_flt++; + else + tsk->min_flt++; + } #ifdef CONFIG_X86_32 /* @@ -909,6 +920,16 @@ do_sigbus: tsk->thread.trap_no = 14; force_sig_info_fault(SIGBUS, BUS_ADRERR, address, tsk); } +#ifdef CONFIG_X86_64 +asmlinkage +#endif +void __kprobes do_page_fault(struct pt_regs *regs, + unsigned long error_code) +{ + current->flags |= PF_FAULT_MAYRETRY; + __do_page_fault(regs, error_code); + current->flags &= ~PF_FAULT_MAYRETRY; +} DEFINE_SPINLOCK(pgd_lock); LIST_HEAD(pgd_list); diff --git a/include/linux/mm.h b/include/linux/mm.h index 72a15dc..4511f68 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -697,6 +697,7 @@ static inline int page_mapped(struct page *page) #define VM_FAULT_NOPAGE 0x0100 /* ->fault installed the pte, not return page */ #define VM_FAULT_LOCKED 0x0200 /* ->fault locked the returned page */ +#define VM_FAULT_RETRY 0x0400 #define VM_FAULT_ERROR (VM_FAULT_OOM | VM_FAULT_SIGBUS) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index bf33413..9d065a4 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -108,7 +108,6 @@ struct vm_area_struct { unsigned long vm_start; /* Our start address within vm_mm. */ unsigned long vm_end; /* The first byte after our end address within vm_mm. */ - /* linked list of VM areas per task, sorted by address */ struct vm_area_struct *vm_next; diff --git a/include/linux/sched.h b/include/linux/sched.h index 3d9120c..bc39432 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1507,6 +1507,7 @@ extern cputime_t task_gtime(struct task_struct *p); #define PF_SPREAD_PAGE 0x01000000 /* Spread page cache over cpuset */ #define PF_SPREAD_SLAB 0x02000000 /* Spread some slab caches over cpuset */ #define PF_THREAD_BOUND 0x04000000 /* Thread bound to specific cpu */ +#define PF_FAULT_MAYRETRY 0x08000000 /* I may drop mmap_sem during fault */ #define PF_MEMPOLICY 0x10000000 /* Non-default NUMA mempolicy */ #define PF_MUTEX_TESTER 0x20000000 /* Thread belongs to the rt mutex tester */ #define PF_FREEZER_SKIP 0x40000000 /* Freezer should not count it as freezeable */ diff --git a/mm/filemap.c b/mm/filemap.c index 876bc59..f9f11bd 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1443,18 +1443,17 @@ int filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf) /* * Do we have something in the page cache already? */ -retry_find: page = find_lock_page(mapping, vmf->pgoff); /* * For sequential accesses, we use the generic readahead logic. */ if (VM_SequentialReadHint(vma)) { if (!page) { + up_read(&vma->vm_mm->mmap_sem); page_cache_sync_readahead(mapping, ra, file, vmf->pgoff, 1); - page = find_lock_page(mapping, vmf->pgoff); - if (!page) - goto no_cached_page; + down_read(&vma->vm_mm->mmap_sem); + return VM_FAULT_RETRY; } if (PageReadahead(page)) { page_cache_async_readahead(mapping, ra, file, page, @@ -1489,7 +1488,10 @@ retry_find: if (vmf->pgoff > ra_pages / 2) start = vmf->pgoff - ra_pages / 2; + up_read(&vma->vm_mm->mmap_sem); do_page_cache_readahead(mapping, file, start, ra_pages); + down_read(&vma->vm_mm->mmap_sem); + return VM_FAULT_RETRY; } page = find_lock_page(mapping, vmf->pgoff); if (!page) @@ -1527,7 +1529,9 @@ no_cached_page: * We're only likely to ever get here if MADV_RANDOM is in * effect. */ + up_read(&vma->vm_mm->mmap_sem); error = page_cache_read(file, vmf->pgoff); + down_read(&vma->vm_mm->mmap_sem); /* * The page we want has now been added to the page cache. @@ -1535,7 +1539,7 @@ no_cached_page: * meantime, we'll just come back here and read it again. */ if (error >= 0) - goto retry_find; + return VM_FAULT_RETRY; /* * An error return from page_cache_read can result if the @@ -1560,16 +1564,18 @@ page_not_uptodate: * and we need to check for errors. */ ClearPageError(page); + up_read(&vma->vm_mm->mmap_sem); error = mapping->a_ops->readpage(file, page); if (!error) { wait_on_page_locked(page); if (!PageUptodate(page)) error = -EIO; } + down_read(&vma->vm_mm->mmap_sem); page_cache_release(page); if (!error || error == AOP_TRUNCATED_PAGE) - goto retry_find; + return VM_FAULT_RETRY; /* Things didn't work out. Return zero to tell the mm layer so. */ shrink_readahead_size_eio(file, ra); --- Second attempt arch/x86/mm/fault.c | 33 ++++++++++++++++++++++++----- include/linux/mm.h | 1 include/linux/sched.h | 1 mm/filemap.c | 56 +++++++++++++++++++++++++++++++++++++++++++++----- 4 files changed, 80 insertions(+), 11 deletions(-) diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c index 455f3fe..38bea4b 100644 --- a/arch/x86/mm/fault.c +++ b/arch/x86/mm/fault.c @@ -578,10 +578,7 @@ int show_unhandled_signals = 1; * and the problem, and then passes it off to one of the appropriate * routines. */ -#ifdef CONFIG_X86_64 -asmlinkage -#endif -void __kprobes do_page_fault(struct pt_regs *regs, unsigned long error_code) +static inline void __do_page_fault(struct pt_regs *regs, unsigned long error_code) { struct task_struct *tsk; struct mm_struct *mm; @@ -702,6 +699,7 @@ again: down_read(&mm->mmap_sem); } +retry: vma = find_vma(mm, address); if (!vma) goto bad_area; @@ -761,8 +759,21 @@ survive: } if (fault & VM_FAULT_MAJOR) tsk->maj_flt++; - else - tsk->min_flt++; + else { + if ((fault & VM_FAULT_RETRY) && (current->flags & PF_FAULT_MAYRETRY)) { + current->flags &= ~PF_FAULT_MAYRETRY; + goto retry; + } + /* + * If we had to retry (PF_FAULT_MAYRETRY cleared), then + * the page originally wasn't up to date before the + * retry, but now it is. + */ + if (!(current->flags & PF_FAULT_MAYRETRY)) + tsk->maj_flt++; + else + tsk->min_flt++; + } #ifdef CONFIG_X86_32 /* @@ -909,6 +920,16 @@ do_sigbus: tsk->thread.trap_no = 14; force_sig_info_fault(SIGBUS, BUS_ADRERR, address, tsk); } +#ifdef CONFIG_X86_64 +asmlinkage +#endif +void __kprobes do_page_fault(struct pt_regs *regs, + unsigned long error_code) +{ + current->flags |= PF_FAULT_MAYRETRY; + __do_page_fault(regs, error_code); + current->flags &= ~PF_FAULT_MAYRETRY; +} DEFINE_SPINLOCK(pgd_lock); LIST_HEAD(pgd_list); diff --git a/include/linux/mm.h b/include/linux/mm.h index 72a15dc..e150c80 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -694,6 +694,7 @@ static inline int page_mapped(struct page *page) #define VM_FAULT_SIGBUS 0x0002 #define VM_FAULT_MAJOR 0x0004 #define VM_FAULT_WRITE 0x0008 /* Special case for get_user_pages */ +#define VM_FAULT_RETRY 0x0016 #define VM_FAULT_NOPAGE 0x0100 /* ->fault installed the pte, not return page */ #define VM_FAULT_LOCKED 0x0200 /* ->fault locked the returned page */ diff --git a/include/linux/sched.h b/include/linux/sched.h index 3d9120c..bc39432 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1507,6 +1507,7 @@ extern cputime_t task_gtime(struct task_struct *p); #define PF_SPREAD_PAGE 0x01000000 /* Spread page cache over cpuset */ #define PF_SPREAD_SLAB 0x02000000 /* Spread some slab caches over cpuset */ #define PF_THREAD_BOUND 0x04000000 /* Thread bound to specific cpu */ +#define PF_FAULT_MAYRETRY 0x08000000 /* I may drop mmap_sem during fault */ #define PF_MEMPOLICY 0x10000000 /* Non-default NUMA mempolicy */ #define PF_MUTEX_TESTER 0x20000000 /* Thread belongs to the rt mutex tester */ #define PF_FREEZER_SKIP 0x40000000 /* Freezer should not count it as freezeable */ diff --git a/mm/filemap.c b/mm/filemap.c index 876bc59..212ea0f 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -670,6 +670,7 @@ repeat: } EXPORT_SYMBOL(find_get_page); +#define NOPAGE_RETRY ((struct page*)-1) /** * find_lock_page - locate, pin and lock a pagecache page * @mapping: the address_space to search @@ -680,14 +681,31 @@ EXPORT_SYMBOL(find_get_page); * * Returns zero if the page was not present. find_lock_page() may sleep. */ -struct page *find_lock_page(struct address_space *mapping, pgoff_t offset) +static struct page *__find_lock_page(struct address_space *mapping, + pgoff_t offset, struct rw_semaphore *mmap_sem) { struct page *page; repeat: page = find_get_page(mapping, offset); if (page) { - lock_page(page); + if(!mmap_sem) { + lock_page(page); + } else if(!trylock_page(page)) { + /* + * Page is already locked by someone else. + * We don't want to be holding down_read(mmap_sem) + * inside lock_page(), so use wait_on_page_locked() here. + */ + up_read(mmap_sem); + wait_on_page_locked(page); + down_read(mmap_sem); + /* + * The VMA tree may have changed at this point. + */ + page_cache_release(page); + goto repeat; + } /* Has the page been truncated? */ if (unlikely(page->mapping != mapping)) { unlock_page(page); @@ -698,6 +716,10 @@ repeat: } return page; } +struct page *find_lock_page(struct address_space *mapping, pgoff_t offset) +{ + return __find_lock_page(mapping, offset, NULL); +} EXPORT_SYMBOL(find_lock_page); /** @@ -1427,6 +1449,8 @@ int filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf) struct address_space *mapping = file->f_mapping; struct file_ra_state *ra = &file->f_ra; struct inode *inode = mapping->host; + struct rw_semaphore *mmap_sem; + struct rw_semaphore *mmap_sem_mayretry; struct page *page; pgoff_t size; int did_readaround = 0; @@ -1435,6 +1459,7 @@ int filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf) size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT; if (vmf->pgoff >= size) return VM_FAULT_SIGBUS; + up_read(&vma->vm_mm->mmap_sem); /* If we don't want any read-ahead, don't bother */ if (VM_RandomReadHint(vma)) @@ -1443,16 +1468,25 @@ int filemap_fault(struct vm_area_struct *vma, struct vm_fault *vmf) /* * Do we have something in the page cache already? */ + mmap_sem = &vma->vm_mm->mmap_sem; + mmap_sem_mayretry = current->flags & PF_FAULT_MAYRETRY ? mmap_sem : NULL; retry_find: - page = find_lock_page(mapping, vmf->pgoff); + page = __find_lock_page(mapping, vmf->pgoff, mmap_sem_mayretry); + if(page == NOPAGE_RETRY) + goto nopage_retry; /* * For sequential accesses, we use the generic readahead logic. */ if (VM_SequentialReadHint(vma)) { if (!page) { + up_read(mmap_sem); page_cache_sync_readahead(mapping, ra, file, vmf->pgoff, 1); - page = find_lock_page(mapping, vmf->pgoff); + down_read(mmap_sem); + page = __find_lock_page(mapping, vmf->pgoff, + mmap_sem_mayretry); + if(page == NOPAGE_RETRY) + goto nopage_retry; if (!page) goto no_cached_page; } @@ -1489,9 +1523,15 @@ retry_find: if (vmf->pgoff > ra_pages / 2) start = vmf->pgoff - ra_pages / 2; + up_read(mmap_sem); do_page_cache_readahead(mapping, file, start, ra_pages); + down_read(mmap_sem); } - page = find_lock_page(mapping, vmf->pgoff); + page = __find_lock_page(mapping, vmf->pgoff, + (current->flags & PF_FAULT_MAYRETRY) ? + &vma->vm_mm->mmap_sem : NULL); + if(page == NOPAGE_RETRY) + goto nopage_retry; if (!page) goto no_cached_page; } @@ -1527,7 +1567,9 @@ no_cached_page: * We're only likely to ever get here if MADV_RANDOM is in * effect. */ + up_read(mmap_sem); error = page_cache_read(file, vmf->pgoff); + down_read(mmap_sem); /* * The page we want has now been added to the page cache. @@ -1560,12 +1602,14 @@ page_not_uptodate: * and we need to check for errors. */ ClearPageError(page); + up_read(mmap_sem); error = mapping->a_ops->readpage(file, page); if (!error) { wait_on_page_locked(page); if (!PageUptodate(page)) error = -EIO; } + down_read(mmap_sem); page_cache_release(page); if (!error || error == AOP_TRUNCATED_PAGE) @@ -1574,6 +1618,8 @@ page_not_uptodate: /* Things didn't work out. Return zero to tell the mm layer so. */ shrink_readahead_size_eio(file, ra); return VM_FAULT_SIGBUS; +nopage_retry: + return VM_FAULT_RETRY; } EXPORT_SYMBOL(filemap_fault); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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